For transmission of data over a predetermined transmission path or recording of data onto recording media such as magnetic discs, optical discs, and magneto-optical discs, the data is modulated so as to become compatible with the transmission path or the recording media. One known modulation method is block coding. In block coding, a data sequence is divided into blocks of m×i bits (the blocks are hereinafter referred to as data words) and each data word is converted into a codeword of n×i bits in accordance with an appropriate coding rule. If i=1, the code is a fixed-length code. If a plurality of i's are selectable or if conversion is performed using a predetermined i selected from a range of 1 to imax (maximum i), the code is a variable-length code. The block-coded code is expressed as a variable-length code (d, k; m, n; r).
As used herein, i is referred to as the constraint length; imax corresponds to r (maximum constraint length); d indicates the minimum number of consecutive “0's” that are inserted between consecutive “1's”, e.g., the minimum run length of “0”; and k indicates the maximum number of consecutive “0's” that are inserted between consecutive “1's”, e.g., the maximum run length of “0”.
When the thus obtained variable-length code is recorded onto an optical disc, a magneto-optical disc, or the like, for example, in the case of a compact disc or mini disk, the variable-length code is subjected to NRZI (Non Return to Zero Inverted) modulation in which “1” is inverted and “0” is not inverted, and recording is performed based on the NRZI-modulated variable-length code (hereinafter also referred to as a recording waveform string). In the initial ISO (International Organization for Standardization) compatible magneto-optical discs having relatively low recording density, modulated recording bit strings which were not subjected to NRZI modulation were recorded.
Assuming that the minimum inversion interval of the recording waveform string is indicated by Tmin and the maximum inversion interval of the recording waveform string is indicated by Tmax, a longer minimum inversion interval Tmin or greater minimum run length d is desirable for high-density recording in the linear velocity direction, and a shorter maximum inversion interval Tmax or smaller maximum run length k is desirable in view of clock playback. A variety of modulation methods meeting these conditions have been proposed.
Specific modulation methods proposed or actually used in, for example, optical discs, magnetic discs, magneto-optical discs, and the like include a variable-length RLL (1-7) (also expressed as (1, 7; m, n; r)) code or RLL (2-7) (also expressed as (2, 7; m, n; r)) code, and a fixed-length RLL (1-7) (also expressed as (1, 7; m, n; 1)) code for use in ISO-compatible MO discs. High-recording-density disc devices such as optical discs and magneto-optical discs which are being developed and studied often use RLL codes (Run Length Limited codes) with minimum run length d=1.
An example conversion table of the variable-length RLL (1-7) code is shown as follows:
TABLE 1RLL(1, 7; 2, 3; 2)DataCodei = 11100x100100110xi = 20011000 00x0010000 0100001100 00x0000100 010
In this conversion table, symbol x corresponds to “1” when the subsequent channel bit is “0”, and corresponds to “0” when the subsequent channel bit is “1”. The maximum constraint length r is 2.
The parameters of the variable-length RLL (1-7) are (1, 7; 2, 3; 2). If the bit interval of the recording waveform string is indicated by T, the minimum inversion interval Tmin expressed by (d+1) T is equal to 2 (=1+1) T. Assuming that the bit interval of the data sequence is indicated by Tdata, then the minimum inversion interval Tmin given by (m/n)×2 is equal to 1.33 (=(⅔)×2) Tdata. The maximum inversion interval Tmax given by (k+1) T is equal to 8 (=7+1) T (=(m/n)×8 Tdata=(⅔)×8 Tdata=5.33 Tdata). The detection window margin Tw is expressed by (m/n)×Tdata, the value thereof being equal to 0.67 (=⅔) Tdata.
In the channel bit strings modulated in accordance with the RLL (1-7) coding shown in Table 1, 2T, namely, Tmin, occurs most frequently, followed by 3T and 4T. In most cases, frequent occurrence of edge information with a short period, such as 2T or 3T, is favorable to clock playback.
In contrast, when the recording density in the linear velocity direction becomes higher, a problem with Tmin arises. Specifically, when the minimum run length, namely, 2T, occurs consecutively, the recording waveform is easily distorted because the waveform output of 2T is smaller than other waveform outputs and is susceptible to, for example, noise, defocusing, tangential tilt, or the like.
For high linear density recording, therefore, recording with consecutive Tmin (2T) is sensitive to disturbance such as noise, and this may lead to errors during data playback. In such a case, data playback errors are often caused by simultaneous shift of the leading edge and the trailing edge of the consecutive Tmin (2T). In other words, the bit error length becomes longer.
In recording data onto recording media or transmitting data, the data is modulated in accordance with a coding scheme compatible with the recording media or transmission path. If the modulated code contains a direct-current component, fluctuation or jitter may be caused in various error signals, such as for tracking error in servo control of a disc device. Therefore, it is desirable that the modulated code contains few direct-current components.
Accordingly, DSV (Digital Sum Value) control has been proposed. DSV is the sum of bits of an NRZI-modulated (namely, level-coded) channel bit string by allocating +1 to the bit string (symbol of the data) set to “1” and −1 to the bit string set to “0”. Reducing the absolute value of DSV, which is a measure of a direct-current component of the code string, that is, performing DSV control, enables suppression of the direct-current component of the code string.
The codes modulated in accordance with the variable-length RLL (1-7) table shown in Table 1 are not subjected to DSV control. DSV control is realized by determining the DSV of the modulated code string (channel bit string) at predetermined intervals and inserting predetermined DSV control bits in the code string (channel bit string).
Basically, the DSV control bits are redundant bits. In view of code conversion efficiency, the fewer the DSV control bits, the better.
Desirably, the inserted DSV control bits do not cause changes in the minimum run length d and the maximum run length k. Changes of (d, k) affect recording and playback characteristics.
Actually, the RLL code must meet the minimum run length requirement, although the maximum run length requirement is not necessarily met. There are some formats in which patterns exceeding the maximum run length are used for a synchronization signal. For example, the 8-16 code for use in DVDs (Digital Versatile Disks) has a maximum run length of 11T, but gives 14T, exceeding the maximum run length, to the synchronization signal pattern portion in order to enhance the detection performance of the synchronization signal.
Accordingly, in the RLL (1-7) coding with improved conversion efficiency, for supporting high density recording, it is important to control the minimum run length so that it becomes more suitable for high linear density and to perform DSV control as efficiently as possible.
For example, Japanese Unexamined Patent Application Publication No. 11-177431, filed by the present applicant, discloses a modulation apparatus including DSV control bit insertion means for inserting a first DSV control bit in a data sequence to generate a first data sequence and for inserting a second DSV control bit in the data sequence to generate a second data sequence; modulation means for modulating both the first data sequence and the second data sequence using a conversion table in which the minimum run length d is 1 and the remainder of the number of “1's” in each element of a data sequence divided by two and the remainder of the number of “1's” in each element of a converted codeword string divided by two are equally 1 or 0; and DSV calculation means for determining a first sectional DSV of the first data sequence modulated using the conversion table and a second sectional DSV of the second data sequence modulated using the conversion table so that the determined sectional DSVs are added to the DSV up to the present position and for selecting and outputting, from the resulting DSV, one of the first data sequence and second data sequence modulated using the conversion table.
FIG. 1 is a block diagram showing the structure of a modulation apparatus of the related art.
As shown in FIG. 1, a modulation apparatus 10 includes a DSV bit insertion unit 11 for inserting “1” or “0”, serving as a DSV bit, into an input data sequence at predetermined intervals. The DSV bit insertion unit 11 has a data sequence containing the DSV bit “1” and another data sequence containing the DSV bit “0”. A modulation unit 12 modulates the data sequence in which the DSV bit is inserted by the DSV bit insertion unit 11. A DSV control unit 13 NRZI-modulates the codeword strings modulated by the modulation unit 12 to obtain level data, determines the DSV of the level data, and finally outputs a DSV-controlled recording code string.
As another example, Japanese Unexamined Patent Application Publication No. 11-346154, filed by the present applicant, discloses a conversion table including, as a conversion code, a basic code where d=1, k=7, m=2, and n=3; a coding rule that the remainder of the number of “1's” in each element of a data sequence divided by two and the remainder of the number of “1's” in each element of a converted codeword string divided by two are equally 1 or 0; a first replacement code for limiting the minimum run length d to a predetermined number or less; and a second replacement code for meeting the run length limitations.
FIG. 2 is a block diagram showing the structure of another modulation apparatus of the related art.
As shown in FIG. 2, a modulation apparatus 20 includes a DSV control bit determination and insertion unit 21 for determining a DSV control bit “1”0 or “0” and for inserting the DSV control bit into an input data sequence at predetermined intervals; a modulation unit 22 for modulating the data sequence containing the DSV control bit; and an NRZI unit 23 for converting the output of the modulation unit 22 into a recording waveform string. The modulation apparatus 20 further includes a timing management unit 24 for generating a timing signal and supplying the timing signal to the parts to manage the timing.
One problem is that the circuit structure of the above-described modulation apparatus is complicated. Another problem is that the complicated circuit structure makes it difficult to apply the apparatus to other systems.